1. Field of the Invention
The invention is related to the field of radio location systems and, in particular, to a system for locating stolen objects or objects removed from the owner's possession.
2. Description of the Prior Art
One of the major problems facing the owner of an object or item having any value is the fact that there is a relatively high probability of it being stolen. In particular, automobiles, trucks, the contents of trucks, aircraft, boats, and various types of off-road vehicles are the primary targets of thieves. These vehicles may be moved a considerable distance in a very short time, making recovery difficult, if not impossible. This problem has been further aggravated in recent years by the advent of the so called "chop shops" in which a stolen vehicle may be disassembled for parts in less than one hour, extinguishing any hope of recovery.
As is known, it is almost impossible to prevent the vehicle from being stolen. Experienced thieves can break into a locked vehicle and drive it away in less than a minute while others have used a tow truck to steal the vehicle. It, therefore, would be advantageous to locate the vehicle as soon as possible after it has been stolen to prevent it from being disassembled in a "chop shop", temporarily hidden for subsequent resale, or transported out of the state. In many of these cases, and in particular in the case of the "chop shop", the owner or the police may have less than an hour to locate the vehicle if there is to be any hope of recovery.
This problem is addressed by the auto theft detection system taught by Reagan in U.S. Pat. No. 4,177,466. In Reagan's theft detection system, each vehicle is identified by a unique code and each vehicle has a transponder responsive to its unique code to transmit a continuous locating signal. A plurality of location stations having proximity detection receivers and/or direction finding receivers, will generate signals indicative of the proximity of the vehicle to a particular location station or the direction of the vehicle from each of a plurality of receivers so that its location can be determined.
Another location system is taught by Jarvis in U.S. Pat. No. 3,665,312. In Jarvis' system, a radio alarm is activated by an illegal entry detector. Upon the detection of an illegal entry, the detector will activate a multiple frequency transmitter to transmit a coded signal which uniquely identifies that particular vehicle. The transmitter will sequentially shift the transmission frequency at predetermined and different time intervals. The transmitted signals are received by a plurality of direction finder receivers which permit the location of the vehicle to be determined.
In addition to proximity detectors and direction finder receivers, various other methods are available for radio location of a transmitting source. Wimbush, in U.S. Pat. No. 4,494,119, discloses a distress radio location system in which remotely located slave repeaters report the strength of a received distress signal to a central station. The central station computes the vehicle's location from the strength of the distress signal at each of the various slave repeaters. GEOSTAR is another type of location system using three remotely positioned satellites. As described in Popular Science, February 1984, pp 76-78 and 130, the location is computed by a ground station from the time a location request signal is received at each of the three satellites. The well known passive LORAN (Long Range Navigation) system is still another method that may be used to determine a vehicle's location.
Although the various location techniques are well known, the utilization of these techniques for the location of a stolen vehicle or object have not been fully exploited by the prior art, nor does the prior art teach a location system in which the transmission times by the vehicle mounted unit are not very short, difficult to detect, and difficult to jam or confuse.